The present invention relates to multichip module assemblies and more particularly to high-density interconnect multichip modules.
Many types of electronic devices employ a number of integrated circuit chips which are interconnected to one another and assembled in a single package or module. Integrated circuits are currently being constructed employing various forms of multichip module (MCM) technology. A multichip module is a chip carrier on which various chips are located and on which the chip terminals are fed out by various means to remotely located connections. Most commonly, these multichip modules are incorporated in multi-layer dielectric substrates that employ thin film processes for forming electrically conductive traces to interconnect the various chips. The multi-layer dielectric substrates are made employing techniques that have been initially developed for various types of semiconductor processing. These MCM""s may include high density interconnect (HDI) technology. In an MCM, many chips may be connected in close proximity to each other.
A different, less costly technology, called low temperature co-fired ceramic (LTCC) has been developed for forming the multi-layer interconnecting circuits for multi-layer electronic packages. LTCC technology employs multiple layers of ceramic tape processed by thick film techniques to provide conductive circuit traces and interconnections extending between the many layers of the LTCC module. The LTCC module is capable of high volume, low cost manufacturing.
Described herein is a multichip module. The multichip module includes a carrier module with at least one cavity formed therein. Positioned within the cavity is at least one submodule assembly which includes at least one electronic element mounted thereon. Covering both the carrier module and the submodule is a layer of flex circuitry which establishes electrical contact with the submodule assembly and provides for the transmission of signals. The layer of flex circuitry is removable in order that the submodule assembly may be removed from the cavity, inspected and replaced.
In one aspect of the invention the submodule assembly is constructed from low temperature co-fired ceramic (LTCC). The LTCC submodule assembly includes at least one electronic component mounted thereon. The electronic component may be any type of integrated circuit chip, for example, a signal processing chip or a microwave chip, or it may be a single electronic component such as a capacitor or a resistor. In lieu of mounting a hard component on the submodule assembly, single components such as capacitors or resistors can be silk screened onto an exterior surface.
The submodule assembly is constructed from multiple layers of an unfired ceramic tape. Before construction begins, it is first determined which electrical components will be mounted on the submodule and what sort of electrical connections are needed. Once this is known, an electric circuit is designed to provide the appropriate electrical connections and silk screens are prepared to apply the circuit to the tape layers. Before the silkscreens are applied, vias are punched through some or all of the layers at locations where electrical connections between layers will be created. The silk screens, which may include a refractory metal paste, may be then applied to the layers to establish the conduction paths. The application of the silk screen also metallizes the vias which provide vertical conduction paths between layers. In the design of the circuit, as many layers as necessary can be used in order to complete the desired circuit.
The ceramic layers have the characteristic of providing electrical insulation between conduction lines which may pass in close proximity to each other on the different layers. Further, an entire layer of the ceramic tape may be metallized in order to create a power source or a ground plane. During the creation of the ceramic tape layers, connection pads are also silk screened onto what will be the exterior surfaces of the submodule assembly. These connection pads will provide electrical connection between elements mounted on the submodule assembly as well as to devices located remote therefrom.
Once all the circuitry has been applied to the individual layers, the entire structure is either held together in a vise type device, or the layers are laminated. The stack of ceramic tape layers is then fired in an oven at a predetermined temperature. At this point the layers of the submodule assembly become a single structure. Once the firing process is complete, the external pads are plated with a conductive metal in order to establish a good electrical connection with elements connected at these points.
Once the above-described processes are complete, the electronic components may be mounted on the submodule assembly. This may be done through at least two different methods. In the first aspect of the invention, the electrical components are mounted on the submodule substrate through establishing wire bond connections between connection pads on the electronic components, and the connection pads which were silk screened on the submodule assembly. In order to install the circuit dice, these elements are aligned so that the connection pad on both elements are in close proximity to each other. In another aspect of the invention, the circuit dice are surface mount assembly (SMA) devices with external connection pads which can be directly soldered to the connection pads on the submodule assembly. In yet another aspect of the invention, individual components such as capacitors or resistors can be silk screened on the submodule assembly such that the resulting components are in electrical contact with the connection pads.
As an alternative to the co-fired ceramics other multichip module assembly technologies may be used in the construction of the submodule assembly. They include MCM-L where the submodules are constructed of plastic laminate-based dielectrics and copper conductors and MCM-D where the submodules are formed by deposition of thin film metals and dielectrics, which may be polymers or inorganic dielectrics.
With construction of the submodule assembly completed, the carrier module is provided for installation. In one aspect of the invention the carrier module may have a cavity formed therein for receiving the submodule assembly. Before installation on to the carrier module, the submodule assembly is flipped such that the electric components mounted on one surface of the submodule assembly are completely enclosed in the cavity. On the opposite side of the submodule assembly connection pads are exposed. Once in the cavity, a layer of flex circuitry is applied over the carrier module and the submodule assembly in order to establish electrical connections.
In one aspect of the invention, the flex circuitry is a combination of an insulative material and a metallization layer. To begin the application process, a layer of insulative material is dispersed over the submodule assembly and the carrier module. In one embodiment of the invention, this insulative material is polyimide. After this layer has been disposed, holes are created in the insulative layer exposing the connection pads on the submodule assembly. A metallization layer is then applied over the entire layer of the plastic insulative material. Application of this metallization layer metallizes the formed holes establishing electrical contact with the exposed connection pads. A circuit has been designed in order to provide the desired electrical connections between the submodule assembly and other electronic components. This circuit is masked off on the metallization layer. The unwanted metallization is etched away, leaving the circuit. In one aspect of the invention, multiple layers of the insulative material and the metallization layer can be applied to create a complex circuit. The insulative material provides electrical insulation between conduction paths which may be in close proximity to each other. In yet another aspect of the invention, the metallization layer may also remain unetched to create a ground plane or be used as part of a power source.
In the event that one of the submodule assemblies needs to be removed, the layer of flex circuitry can be peeled back from the carrier module exposing the submodule assembly. At this point the submodule assembly may be removed, inspected, tested or otherwise disposed of. A new submodule assembly may be then installed and the electrical connections reestablished.